practice of Messrs. Boulton and Watt was generally employed; it was very similar to the old Admiralty rule for paddle steamers, the same effective pressure of 7 lbs. per square inch of piston area being assumed; but the Assumed speed of piston = 128 Length of stroke. This rule has not yet fallen into disuse, but is sometimes stated as follows:-Let D2= sum of squares of diameters of cylinders (in inches); then 1 Nominal ×D× length of stroke. The commercial nominal horse-power is, however, very frequently represented by the following expression— = 1 nominal horse-power 30 circular inches of piston area. A "circular inch" being a circle of 1 inch diameter, the total nominal horse-power of a set of engines would be obtained by finding the number of circular inches in all the piston areas, and dividing by 30. This rule corresponds with that of Messrs. Boulton and Watt, when the piston speed is assumed to be 200 feet per minute. Various proposals have been made with a view to improving the commercial method of measuring horse-power, but none of them has found general favour.* In 1872, the council of the Institution of Naval Architects, having been consulted on the subject by the Board of Trade, replied as follows:-"The term nominal horse-power as at present "ordinarily used for commercial purposes conveys no definite "meaning." "The majority of the committee were of "opinion that no formula depending upon the dimensions of any parts of the engines, boilers, or furnaces could be relied * Mr. McFarlane Gray, of the Board of Trade, proposed a method of measuring nominal horse-power, which was referred to the council of the Institution of Naval Archi tects in 1872, but not approved by them. It was based on two measure. ments, diameter of cylinders and width of furnace. 66 66 'upon as giving a satisfactory measure of the power of an 'engine; and that even if the varieties of engines and boilers "now in use could be comprised under one general expression "for the power, the progress of invention would soon vitiate "any such expression or formula." The committee could not agree to any alternative mode of measuring engine-power, but the plan which met with least objection was to take either the indicated power on a trial trip as the nominal power, or some sub-multiple, such as one-fourth of the indicated power; the latter would be very nearly the same as the French rule. So far as we are informed, no action has yet been taken to give effect to the recommendations, and to assign a uniform or definite meaning to a nominal horse-power in the mercantile marine. In selecting the type of engine to be employed in a new ship, in consultation with the marine engineer, the designer has to consider the ratio of the weight of the various types to their indicated horse-power, and their relative coal consumption. It is usual to express the weight of machinery in "hundredweights per indicated horse-power" and the coal consumed in "pounds per indicated horse-power per hour." Both these quantities may be affected by the special conditions to be fulfilled in various ships, especially in warships, even for any single type of engine; but the following brief statement may be of service, representing, as it does, the average results of good practice in the Royal Navy. Three types of engines are now in common use: first, the ordinary type, with jet condensers, such as is fitted in the Warrior and other early ironclads; second, the surfacecondenser type, such as is fitted in the Hercules, Devastation, and other ironclads built in 1863-71; third, the compound type, that has been extensively adopted during the last few years, and was recommended strongly by the Committee on Designs for Ships of War in their report of 1871. Besides these three types of engines, there are others respecting which nothing need be said. For the three types named, the average weights and rates In the mercantile marine, the compound engine has been extensively used for many years past with the greatest advantage as regards economy of fuel and efficient performance.* It appears, however, that, while the rate of consumption of coal is about 2 lbs. per indicated horse-power per hour when these vessels are making long passages at full speed, the weight of the machinery in proportion to the indicated horse-power is somewhat greater than in the engines of similar type used in the Royal Navy. On the other hand, marine engines have been constructed for special services, with weights not exceeding 2 cwt. per indicated horse-power; and still more in contrast with the general practice is that followed by the builders of the fast steam-launches, in which the weight of the whole propelling apparatus, in full working order, has been brought to about cwt. per indicated horsepower. The remarkable performances of these small vessels. will be further examined hereafter; but it is a question for the marine engineer whether, on the large scale, any similar economy of weight may not be practicable. A simple example will illustrate the advantages gained by adopting a type of engine which economises fuel, even if the engine itself has to be made somewhat heavier. Her Majesty's ship Devastation has engines of the surface-condensing type, which indicated on trial more than 6600 horse * See the appendix to the report of the Committee on Designs for Ships of War, for the records of experience of several eminent marine engineers. power, and drove the ship 138 knots per hour. These engines weigh 1000 tons, and the total coal supply carried at the normal draught is 1350 tons. Had the engines been made on the compound type, they would have weighed about 1250 tons, or 250 tons more than their actual weight; but the rate of coal consumption would have been only twothirds as great as that of the present engines, and therefore, with 900 tons of coal, the vessel would have been able to steam as far as she now can with 1350 tons. This saving on the coals would not only cover the additional weight of the engines, but enable either 200 tons to be added to the weight of armour, armament, and equipment, or the distances over which the ship could steam to be increased about one-fifth, or any corresponding change to be made that might be preferred. In the case of a merchant steamer making frequent passages over a known distance, savings in coal consumption are even more important. One of the large Transatlantic steamers, for instance, now burning 800 tons of coal on the voyage, would, with engines of the old jet-condenser type, have to burn 1800 or 2000 tons, and, with the surface-condenser type of engine, would burn about 1200 or 1400 tons. On the work of a year the savings effected by using the most economical type would be very considerable. The longer the voyage and the larger the proportionate coal supply, the greater are the gains of the modern type. For example, a steamer which now has to carry a weight of coal equalling three-tenths of her total displacement, in order to perform the voyage to Australia, might have nearly one-fourth of the displacement available for cargo. But if she had engines of the early type, consuming coal twice as rapidly, she would require to carry coals amounting to threefifths of her total weight, and could carry no cargo. If she had engines of the surface-condensing type, the coal supply would have to be increased to nearly one-half the displacement; and after allowing for the small saving on the weight of engines, as compared with the compound type, the weight of cargo that could be carried would be very small-not one half that which the modern ship would carry. These are not mere estimates, but simple statements of fact based upon the particulars of ships now employed upon the service. And it is to the improvements in marine engines, which have brought about such great economy in consumption of fuel, that the moderate size of these successful ships is due. When the design of the Great Eastern was in contemplation, no such results had been attained, and it appeared necessary to build a ship of extraordinary dimensions, for a service which is now successfully accomplished by ships of less than one-fourth her displacement. Adopting the indicated horse-power as the fairest measure of the power of engines, it is important to ascertain the ratio which the indicated power bears to the "effective horsepower" previously defined. This ratio will mainly depend upon three circumstances: (1) the efficiency of the mechanism of the engines; (2) the efficiency of the propeller; (3) the increased resistance, due to the changes produced by the action of the propeller in the motion of the water relatively to the hull of the ship. When an engine is in motion under its load, a considerable part of its indicated power must be expended in overcoming frictional and other resistances, working the air-pumps, &c.; and only the remaining part of the power is available to give motion to the propeller. The proportion of the indicated power expended in this "waste work" no doubt varies in different engines, and in any engine when working at different speeds. General experience appears to have shown that one-fourth or one-fifth of the indicated power would be a fair average allowance for the waste work of engines working at full speed; these are the figures given by Professor Rankine and other authorities. The best and most recent information of the kind yet published is, however, due to the researches of Mr. Froude.* Having analysed See his paper "On the Ratio of Indicated to Effective Horse-power," vol. xvii. of the Transactions of the Institution of Naval Architects. |